about select performance

1.our system have a daily report table(produce every day ,have a index in it ) ,  we analyse the table staticts in every night (after 22:00) .

so the next day ,the query's performance will be better,

2.sometime , we will need query the dialy table in the same day afternoon,sometime   we will find  the query  select the table performance will be worse , because it do a  full scan (db file scattered read ), but  after a while , the same

query will be better , use  index rang scan (db file sequential read), so in this situation , if we want avoid the  worse  query plan, how we should do ? use hint ?